The present invention relates to a broad range of processes, including those for the enhancement and modification of objects, such as substrates, as well as to the fabrication of object structures. For example, it is known to coat a substrate to enhance the utility of the substrate. By way of further example, it is known to construct a physical representation of a three-dimensional object stored in a computer memory. These and additional fields of endeavor are discussed in greater detail hereinbelow.
Commercially and economically enhancing, modifying, and/or fabricating objects having a predetermined composite material is highly desirable in many industries, including mechanical and electronic industries. For example, treating a substrate (e.g. applying a diamond or a diamond-like carbon coating) may be desirable to enhance the performance or to expand the applications of the original object. Also, fabricating an object (e.g. rapid prototyping) may be desirable to evaluate an object prior to production.
Prior art methods and apparatus for treating substrates and/or fabricating objects are, however, limited in their application. Although the industry is attempting to overcome these limitations, prior art technologies have struggled to establish an effective process, having a wide range of capabilities, to commercially and economically treat and/or fabricate an object having a desired composite material (e.g. diamond or DLC).
The following is a discussion illustrating the demand for treatment of a surface in the engineering component industry. However, it should be appreciated that these demands also exist in other industries, such as semiconductor packaging and fabrication, surgical, home appliance, and other industries where it may be desirable to improve the performance of an existing object.
Manufacturing processes, for example cutting, stamping, forming have long been an exemplary area of interest for coating the surface of a relatively soft tool with a relatively hard coating material. The development and application of scratch resistant coatings to softer underlying substrates is another exemplary area of prolonged endeavor. The development and application of low-friction coatings is yet another exemplary area of prolonged endeavor. Other areas of interest are in machining, tooling, electronic components, and surgical components.
One area of particular interest is the application of diamond and diamond-like carbon (DLC) to machine tools (e.g., to machine tool cutting inserts). Diamond and DLC are especially tough (hard) materials, wear well, and have thermal qualities which are beneficial in many applications. For many machining applications, the qualities of diamond or DLC are unsurpassed by any other available material. The application of a diamond or DLC to a cutting tool is discussed herein as exemplary of the state of the art in substrate coating technology and as an example of broad range of uses to which the present invention is applicable.
Metal-cutting, or machining, is performed with tools operating on a workpiece. Typically, a tool is rotated and brought to bear upon a workpiece to remove material from the workpiece. The tool may be provided with integral cutting edges, or it may be formed as a carrier for cutting inserts, such as carbide inserts, having one or more cutting edges. Among the numerous factors and considerations that typically are taken into account when machining, more particularly in the choice of tools for machining, are the nature of the material (workpiece) sought to be machined, as well as the following parameters relating to the tool itself:
"tool speed" (typically measured in meters-per-minute, or in feet-per-minute); PA1 "feed rate" (typically measured in millimeters(or inches)-per-revolution); PA1 "depth of cut", or "DOC" (typically measured in millimeters and inches); PA1 "length of cut" (typically measured in millimeters, or in inches); and PA1 lubrication (e.g., versus "dry" machining). PA1 resulting surface finish (typically measured in .mu.in) that will be achieved on the workpiece; PA1 tool life; PA1 abrasion resistance of the tool; PA1 thermal conductivity of the tool; PA1 chemical and thermal stability of the tool; and PA1 tool coefficient of friction. PA1 delamination (catastrophic failure); PA1 adhesive and abrasive wear resistance (diamond is often used as a milestone for evaluating wear resistance); PA1 toughness (carbide is often used as a milestone for evaluating toughness); PA1 flank wear; PA1 Built Up Edge (BUE) heat; and PA1 edge integrity. PA1 there is a need to significantly elevate the temperature of the substrate. PA1 there is a significant (2-5 hour) cooling time, during which time residual precursors (gas or evaporated target materials) deposit, like snowflakes, on the surface being coated. This results in a coating which has a very rough surface, as compared to the pre-coated surface, and which typically requires post-processing to achieve a smoother surface. PA1 when depositing a diamond or DLC coating, an amorphous coating is typically formed, containing either SP.sup.2 -bonded carbon or SP.sup.2 -bonded carbon and SP.sup.3 -bonded carbon, with higher concentration of hydrogen. PA1 both CVD and PVD processes are directed to depositing a material on the surface of a substrate, and rely on molecular bonding. PA1 PVD coatings tend to be porous. CVD coatings tend to be somewhat less porous than PVD coatings. PA1 both CVD and PVD processes are generally limited in suitability to coating flat surfaces, or simple (non-complex geometry) round surfaces. PA1 the size of the substrate that can be coated is limited by the size of the vacuum chamber in which the process is carried out; the size of the substrate is typically less than eight inches in diameter. PA1 inasmuch as these processes tend to rely primarily on a precipitation-type (i.e., generally directional deposition) mechanism, the "other" side of the substrate may exhibit shadowing or uneven deposition. PA1 (a) The CVD or PVD deposition rate is limited to approximately 0.5 .mu.m-10 .mu.m per hour. PA1 (b) The diamond or DLC coating exhibits poor adhesion (e.g., 30 kg/mm.sup.2) on carbide substrates with higher cobalt content, requiring specialty substrates or other surface treatment. PA1 (c) The processes are generally directed to the formation of amorphous diamond-like coatings only, containing SP.sup.2 and/or SP.sup.3 and non-diamond carbon phases (e.g., graphite). PA1 (d) The CVD process requires the substrate to be heated to 450.degree.-1000.degree. C. (degrees Celsius), to enable coating growth and bonding, which can distort the substrate and which can add significant on-line time (e.g., 2 hrs.) to the process. PA1 (e) The processes must be performed in a vacuum chamber, such as a belljar, which adds complexity to the process and which severely limits the size of the substrate to be coated. PA1 (f) The part (substrate) being coated is held stationary, and selective coating can only be achieved by masking the substrate. Also, the processes are directed to forming uniform thickness coatings. PA1 (g) Stainless steel cannot be easily coated using these processes. PA1 (h) Steel cannot be easily coated using these processes. PA1 (i) These processes do not work well for coating the inside diameters (ID, bore) of tubes, other than those having a relatively low length:diameter (L-to-D) ratio. PA1 (j) Post-finishing steps are required for some applications, such as for sculpting and texturing the coatings and providing precision engineering dimensions. PA1 (k) The processes are generally homoepitaxial. PA1 (l) A Raman variance will be exhibited with various substrates, in proportion to the impurity of the coating. (Generally, the Raman effect is the scattering of incident light by the molecules of a transparent substance in such a way that the wavelengths of the scattered light are lengthened or shortened. More specifically, a quantum of light gives up some of its energy to a molecule and reappears as a scattered quantum with a lower frequency.) PA1 (m) The processes are generally not well-suited to coating large surfaces, or surfaces with complex geometries. PA1 (n) process the component by abrading, chemical etching, or other means to nucleate or seed the area to be treated; and PA1 (o) diamond separated from a substrate is under stress and may require further processing for certain applications. The diamond layer removed from the substrate may have internal stresses, and may be deformed. PA1 wavelength (L.sub.1, L.sub.2, L.sub.3); PA1 mode (e.g., pulsed, super-pulsed or continuous wave), including pulse width and frequency; PA1 output power (P1, P2, P3); and PA1 energy (J1, J2, J3). PA1 a first of the three lasers is a pulsed excimer laser, operating at either 192 nm, 248 nm, or 308 nm, with a power output of tens of (0-200) watts (W), with a pulse energy of up to 500 mJ (millijoules), a pulse length of up to 26 nanoseconds (ns), and a repetition rate of up to 300 Hz (Hertz); PA1 a second of the three lasers is a Nd:YAG laser, operating at 1.06 microns in a continuous (CW) or burst mode, or Q-switch with a power output of hundreds of (0-1500) watts, with a pulse energy of up to 150 J (Joules), a pulse frequency of up to 1000 Hz, a pulse length of up to 20 milliseconds (ms), and (in a pulse/burst mode) a pulse stream duration of up to 5 seconds; PA1 a third of the three lasers is a CO.sub.2 laser operating at a wavelength 10.6 microns, with a output power on the order of 500-10000 W, a pulse frequency up to 25 KHz, a pulse up to 25 microseconds, a super-pulse frequency up to 20 KHz, and a super-pulse width up to 500 microseconds. PA1 the substrate is carbon steel; PA1 the constituent element of interest is carbon; PA1 the secondary element, if utilized, may be carbon, depending on the treatment, coating thickness desired, and whether the substrate is high carbon steel or low carbon steel; PA1 the resulting conversion zone depth "d" is approximately 1.0 mm (including an approximately 0.25 mm secondary conversion zone); and; PA1 the resulting diamond coating thickness "t" is approximately 3 mm (or approximately three times the depth of the conversion zone). PA1 providing a continuous reaction system at selected (discrete) areas of the substrate PA1 a fabricated composition may be either "truly heteroepitaxial" and/or homoepitaxial; for example, a fabricated heteroepitaxial fabricated composition may develop into a homoepitaxial fabricated composition (e.g., the coating, or a subsequent coating when the composition is SP.sup.3 carbon-bonded). The techniques of the present invention allow for growth of a material on another underlying material without limitation as to crystal orientation, lattice structure, direction of growth, materials, etc. In other words, the material being fabricated is not limited (unrestrained) by properties of the material or substrate upon which it is being fabricated. PA1 any lattice structure may be formed as a coating on the surface by choosing an appropriate nucleation material and causing an appropriate species in the material of the substrate to enter the Preliminary Vapor Phase (PVP); PA1 the process may be performed without CVD processes, without a vacuum, and without a target material; PA1 the process may be performed in ambient atmospheric conditions; PA1 the process may be performed without preheating the substrate; PA1 the process is continuous, and allows a composition of any desired depth to be formed below the surface of the substrate, and a composition of any desired thickness to be formed on and above the surface of the substrate; PA1 the bonding is deeper and provides for greater adhesion than the prior art. PA1 the coating can be formed on a substrate of virtually any size and shape, including very large substrates. There is virtually no limit to the thickness or area of composition formed by the process; PA1 treating a substrate to form a diamond or DLC surface maybe accomplished without affecting the original volume of the substrate. PA1 better adhesion; PA1 efficiently achieve greater thickness with growth rates on the order of that achieved by HPHT prior art processes;
Additionally, the following factors are important considerations when machining (e.g., drilling, reaming, milling, end-milling, surface-finishing) a workpiece:
Carbide has long been an established choice for use in cutting tools and inserts, especially for cutting (machining) ferrous, nonferrous or abrasive materials such as aluminum and its alloys, copper, brass, bronze, plastics, ceramics, titanium, fiber-reinforced composites and graphite. Various forms of carbide are known for tools and inserts, such as cobalt-consolidated tungsten carbide (WC/Co).
In recent years, polycrystalline-diamond (PCD) brazed-tip cutting tools have demonstrated their feasibility. These PCD cutting tools generally offer only one cutting edge, and require relatively high temperature and pressure processes for their fabrication. PCD tool inserts may have a relatively thick diamond layer (2.5 mm is a common thickness). A shortcoming of PCD is that the cobalt binder of a PCD tool can react chemically with certain work materials. PCD appears to be beneficial for the machining of high-silicon aluminum alloys and other highly abrasive materials.
In certain applications, however, for example in the machining of carbon phenolic composite material, uncoated carbide and polycrystalline diamond tools (PCD) have proven to be unsatisfactory. Aluminum oxide (as opposed to diamond) inserts have been shown to be more effective for machining this particular material.
More recently, thin-film-diamond-coated inserts, and thick-film-diamond brazed-tip tools are being developed for machining applications. Generally, a "thin" film is a film that is less than 100 .mu.m (micrometers) thick.
An example of a thick-film-diamond brazed tool, using chemical vapor deposition (CVD), is the "DT-100" (from Norton Diamond Film of Northboro, Mass.), wherein a diamond or DLC is grown on a disc having diameter of eight inches or less, then laser cut to shape, and then brazed to a carbide shank.
An example of a thin-film-diamond coated carbide insert is the "DCC" (from Crystallume, Menlo Park, Calif.), which has demonstrated a tool life 10-15 times greater than conventional coated carbide in applications turning highly abrasive 390 alloy aluminum. This diamond-coated insert is made by a chemical vapor deposition (CVD) process, and is reported to outperform uncoated carbide and perform equally to or better than PCD tools. The CVD process employed is reportedly a microwave plasma enhanced (MPE) CVD process which takes place at relatively low temperatures and pressures (as compared with conventional PCD fabrication methods which utilize High Pressure and High Temperature ("HPHT") techniques. Using these processes, any insert shape can reportedly be uniformly coated, and the coated inserts can have sharp edges and chip-breaker geometries. Hence, these inserts are indexable and can provide from two-to-four cutting corners. CVD-coated tools tend to have a thin diamond layer (typically less than 0.03 mm), which tends to allow the toughness of the underlying substrate material to dominate in determining overall tool strength, even when shock-loaded. Hence, these CVD (e.g., "DCC") inserts tend to be able to handle a larger DOC.
For example, a 30% carbon phenolic composite material has successfully been machined using thin-film-diamond-coated silicon nitride inserts.
A critical concern with any coated tool or insert is that the coating should exhibit good adhesion to the underlying base material (e.g., carbide). Concerns with the prior art include:
Whatever coating is used should additionally be compatible with the material contemplated to be machined. For example, PCD tends to have a very low corrosion resistance to the resins in certain composite plastics.
Another area of concern with respect to diamond or DLC coatings on tools is that a very hard diamond or DLC coating on a softer tool is very prone to failure from stress.
An area of paramount concern is poor adhesion, which would appear to be a result from the reliance of prior art diamond or DLC coatings on the mechanism of molecular bonding and instabilities inherent in formation of diamond or DLC in prior art processes.
As discussed hereinabove, diamond or DLC coatings have been demonstrated to be of value for tool inserts, and Chemical Vapor Deposition (CVD) processes appear to be a popular process for applying such diamond or DLC coatings.
An example of a CVD coating process is growing diamond by reacting hydrogen and a hydrocarbon gas, such as methane, in a plasma and synthesizing a diamond structure either as a coating or a free-standing blank. Carbide tools may be coated with a thin film of diamond using closed-chamber arc plasma CVD.
There are a number of basic CVD deposition processes currently in use, for depositing diamond coatings. Generally, these processes involve dissociation and ionization of hydrogen and methane precursor gases, which are then passed over and deposited onto a heated substrate.
The need to heat the substrate in order to apply the coatings is, in many ways, counterproductive. Such application of heat to the entire substrate can cause distortion of the substrate, and the loss of any temper (heat treatment) that had previously been present in the substrate.
For example, in the heated filament CVD method, a tungsten or tantalum filament is used to heat the precursor gases to about 2000.degree. C. Substrate temperature ranges from 600.degree.-1100.degree. C. Using hydrogen and methane precursors, deposition rates of 1-10 .mu.m per hour are possible.
In DC plasma CVD, a DC (direct current) arc is used to dissociate the precursor gases, and can provide higher gas volumes and velocities than other prior art processes.
Microwave CVD uses microwaves to excite the precursor gases, resulting in deposition rates of several microns per hour. Coatings deposited using this method are of very high purity, closer to pure diamond than the other techniques.
Another coating process, related to CVD, is Physical Vapor Deposition (PVD). In PVD, a target in a vacuum chamber is evaporated, as opposed to introducing a gas to the vacuum chamber with CVD. In both the CVD and PVD processes:
Irrespective of the process involved in applying a coating to a substrate, the end-product may still provide unacceptable results. For example, applying a thin hard coating over a soft substrate will result in very poor stress distribution.
Prior art coating processes tend to be limited to forming a thin film (or layer) on a substrate. This is somewhat analogous to rain falling on a lawn and freezing. The resulting ice layer is relatively hard, but is thin, and there is an abrupt transition of hardness from the thin ice layer (coating) to the underlying grass (substrate). This will result in extremely poor stress distribution, as a result of which the thin layer of ice is subject to cracking when stress is applied. Generally, the thickness of the coating will reflect upon the stresses that build up in the coating.
CVD and PVD diamond or DLC are typically grainy, although they can be post-process finished to provide a surface of desired smoothness. However, in order to perform such post-finishing, a diamond must be employed. Further, as in any abrasive process, there will be directional scratches, albeit microscopic, evident in the final surface finish of the coating.
Coating a tool (or insert) with diamond or DLC has been discussed extensively hereinabove. Diamond is a material of choice for coating tools because of its extreme hardness (9000 kg/mm.sup.2) and its low coefficient of friction (0.05). However, regardless of the substrate material (e.g., cemented carbide) adhesion of diamond or DLC coatings has been a barrier to its widespread application. In the case of carbide substrates, these adhesion problems are augmented by the cobalt binder phase found in carbide tools which essentially "poisons" the diamond nucleation and growth process, resulting in formation of graphitic carbon (which is undesirable).
Attention is directed to the following U.S. Patents, incorporated by reference herein, as indicative of the state of the art of diamond coating: U.S. Pat. Nos. 5,273,790; 5,273,825; 5,271,971; and 5,270,077 ('077). The '077 patent, for example, discloses contacting a heated substrate with an excited gaseous hydrogen and hydrocarbon mixture under conditions of pressure, temperature and gas concentration which promote the growth of a diamond coating on a convex growth surface of the substrate, then separating the diamond coating from the convex growth surface, to provide a flat diamond film. The diamond coating separated from the substrate is under stress, and may require further processing for certain applications. Due to internal residual stresses, the diamond layer may also be deformed.
Generally, the prior art techniques for applying diamond or DLC coatings to a substrate (e.g., tool insert), although useful, suffer from one or more of the following limitations (a-O):
These (a-o) and other limitations of the prior art are addressed by the techniques of the present invention, which do not depend upon a vacuum environment, and which do not require preheating the substrate to perform the coating. Generally, the techniques of the present invention are not dependent on CVD or PVD processes, and the results obtained are superior in many respects to those processes. The techniques of the present invention may be used to enhance prior art processes.
An example of a non-CVD process is found in U.S. Pat. No. 5,273,788, entitled PREPARATION OF DIAMOND AND DIAMOND-LIKE THIN FILMS, incorporated by reference herein. This patent discloses a method for forming a diamond-like carbon (DLC) film on a substrate, and generally involves the steps of depositing a Langmuir-Blodgett (LB) molecular layer of a molecule containing carbon and hydrogen on the surface of the substrate, and irradiating the molecular layer with a laser beam sufficient to reform bonds between carbon atoms in the molecular layer, so as to form a DLC film on the substrate. The Langmuir-Blodgett (LB) technique involves forming one or more molecular layers in an ordered array consisting of surfactant-type organic molecules with a hydrophilic polar head group and a hydrophobic tail. It is represented in this patent that the laser irradiates and carbonizes the LB layer (decomposes the organic molecules in the LB layer), "but has little or no effect on the substrate (itself)". The laser power and time of irradiation is chosen to induce rebonding of the carbon atoms in the LB layer and achieve formation of a DLC film on the substrate. The process of this patent permits the formation of DLC films without a vacuum chamber, and the DLC films may contain minor amounts of other elements or substances that do not interfere with the formation or function of the DLC film, such as oxygen from air that is bonded to the carbon during irradiation. This patent makes reference to other techniques for forming diamond films by immersing a substrate in a fluid medium comprising a carbon-containing precursor and irradiating the substrate with a laser to pyrolize the precursor. One example that is given is a process (U.S. Pat. No. 4,948,629) for the deposition of diamond films where gas containing an aliphatic acid or an aromatic carboxylic anhydride that vaporizes without decomposition is passed over a substrate and irradiated with a focused, high-powered, pulsed laser. Another example that is given is a process (U.S. Pat. No. 4,954,365) where the substrate is immersed in a liquid containing carbon and hydrogen (e.g., methanol), a laser pulse is directed through the liquid coating to heat the substrate, the liquid is pyrolized, and carbon material from the pyrolized liquid grows on the substrate to form a diamond coating on the substrate. Additional examples of processes employing lasers to form diamond films are set forth in this patent, which is primarily directed to pyrolizing a LB layer as a precursor to a completely reformed film.
U.S. Pat. No. 4,849,199, incorporated by reference herein, discloses a method for suppressing growth of graphite and other non-diamond carbon species during formation of synthetic diamond. As is noted in the patent, high pressure processes for synthesizing diamond all tend to suffer from the growth of graphite, which eventually causes diamond growth to cease. A low pressure method is disclosed in the patent whereby growth of graphite and other non-diamond carbon species is suppressed by evaporation or selective photolysis. In one method disclosed in the patent, the graphite or other non-diamond carbon species is vaporized using incident radiant energy sufficient to vaporize graphite but insufficient to damage the substrate. In another method disclosed in the patent, the graphite or other non-diamond carbon species is selectively photolyzed, such as, by the use of laser energy of appropriate wavelength. The methods of the patent are intended to function in conjunction with a plasma enhanced chemical vapor deposition process (PECVD) to grow diamonds on seed crystals, requiring a carbon source gas. The use of a laser is suggested to vaporize the graphite and non-diamond carbon species as they form upon the diamond growing surface, with the caveat that the laser energy should be low enough to avoid any substantial physical or chemical damage to the substrate, particularly if the substrate is other than a diamond seed crystal. It is further suggested in the patent that control of graphite growth over a large diamond crystal or substrate area may be achieved by scanning a tightly focused beam over the entire area.
U.S. Pat. No. 4,987,007, incorporated by reference herein, discloses a method and apparatus for producing a layer of material from a laser ion source. The process is intended for use in a vacuum environment, and can produce a diamond-like carbon layer of exhibiting uniform thickness with less than 3% variation at a rate of 20 .mu.m per hour. The process involves focusing a laser beam on a target, ablating a portion of the target to eject a plasma of the target substance, ionizing part of the plasma with the laser, and positioning a substrate to collect the ions to form a layer of material on the substrate.
U.S. Pat. No. 4,701,592, incorporated by reference herein, discloses a process of depositing a layer of a material on a substrate and annealing the deposited layer. The process is laser assisted, and proceeds in a vacuum.
The following illustrates prior art methods for fabricating an object (e.g. Rapid Prototyping). Attention is directed to the following U.S. Patents, incorporated by reference herein, as indicative of the state of the art of stereolithography and object fabrication: U.S. Pat. No. 5,260,009 ("System, Method, And Process For Making 3D Objects"); U.S. Pat. No. 5,256,340 ("Method Of Making A 3-D Object By Stereolithography"); U.S. Pat. No. 5,248,456 ("Method And Apparatus For Producing Stereolithographically Produced Objects); U.S. Pat. No. 5,247,180 ("Stereolithographic Apparatus And Method Of Use"); U.S. Pat. No. 5,236,637 ("Method And Apparatus For Production Of 3-D Objects By Stereolithography).
Prior art methods for rapidly making an object provides limited engineering evaluation and are not suitable for production use or prototyping. Prior art technology uses photopolymers or extruded materials, among other non-metallic techniques, to produce rapid prototype plastic parts or laser sintered powders to produce metal parts. All of these methods produce relative rough parts of limited utility.
For example, a stereolithography apparatus (SLA) is typically used in rapid prototyping system. Stereolithography is a process by which three dimensional objects are fabricated from thin layers of hardened cured liquid polymers. Current rapid prototyping systems make an object by selectively hardening or cutting layers of material into a shape defined by CAD data. Typically, ultraviolet, argon-ion, or other type of laser is used harden the polymer. The CAD data mathematically represent the shape of the object to be produced as a series of sequential thin layers.
Several publications have emphasized the importance that rapid-prototyping (RP) technologies will have towards improving manufacturing systems and reducing costs. Furthermore, these articles identify the limitations that exist with the current art. In two recent publications, Manufacturing Engineering (Published by SME, pp. 37-42, November 1993) and Plastics Technology (pp. 40-44, January 1994), the respective authors emphasize the magnitude of rapid prototype and manufacturing systems. Other articles, such as those related to laser sintering, are also indicative of the state of the art in rapid-prototyping.
Prior art methods for fabricating objects are typically used only for rapid prototyping shapes, defined by a CAD program, from a material. Prior art rapid prototyping makes an object by selectively cutting layers of material into a shape defined by the CAD data. As noted in Manufacturing Engineering (November 1993), the "goal of current RP [rapid prototyping] technologies is prototype materials that provide higher strength at elevated temperatures. The industry desires full metal molds (without using sintered materials) so as to effectively analyze the object." Furthermore, this article notes that producing parts directly will be the ultimate step in rapid prototyping. Also, the article emphasized that the key will be materials and that although some experimental rapid prototyping systems are working with molten metals and metal powders, they are still far from high-strength, fully dense metals.
Therefore, what is needed is a method and apparatus for commercially and economically treating and/or fabricating objects to obtain a desired composite material. Furthermore, it is desirable to improve the deposition rate, bond strength, adhesion, process time, area of growth, and material strength. Furthermore, it is desirable to produce an object having a desired composite material so as to permit effective engineering evaluation of material strength and production of parts and produce parts that exhibit surfaces having precise dimensions according to engineering data.